Paul,
The short answer is "No, the process is not the same in western longitudes." The good news is that it is some what easier to do and less confusing.
The Greenwich Hour Angle (GHA) of the body is how far west the body's geographic position (GP) was from the Greenwich meridian at the time of the sight.
Your Dead Reckoning (DR) longitude is (approximately) how far eastward or westward you were from the Greenwich meridian at the time of the sight.
The Local Hour Angle (LHA) is how far westward the body's GP was from your longitude, or from your nearby chosen AP's longitude.
All "Hour Angles" of any sort are always only measured westward from their starting point all the way around to 360°
If you are east of Greenwich you have to add your eastern longitude to the GHA of the body to compute how far westward the body's GP was from yourself. Additionally, just for using tables or taking an exam, the LHA result has to come out to be a whole number of degrees. To work that out we first find the GHA of the body at the time of the sight. Then we subtract the minutes and tenths of the GHA that we found from 60.0' to find a value for the minutes and tenths to append to our DR whole degree of longitude to create an AP longitude that will make the addition and the resulting LHA come out to be a whole number of degrees. We know full well that we are not at the AP longitude that we then use. We only contrived it in order to make the tables work -- or to get the same answer as on the exam.
When you are in western longitudes you will be subtracting your longitude from the Greenwich Hour Angle to determine how far westward the GP of the body was from you at the time of the sight. Once again, even in western longitudes, for using the tables or taking and exam, you need the result to come out to be a whole number of degrees. But this time all you need to do is to take the minutes and tenths of the GHA as you found it and append that value unchanged to your whole degree of DR longitude to contrive your AP longitude. Now when you subtract the longitude of your AP position from the GHA of the body to find your LHA the minutes and tenths will disappear to 00.0' leaving you with a whole degree result.
The final check (only really important for taking exams) is to confirm in either case that your resulting AP longitude is within 30.0' of longitude from your DR position, and if not to adjust the whole degree value of your AP by one degree while leaving the minutes and tenths alone so that it is -- just so your results will match the exam question answer.
Hope this helps -- let me know if you have more questions.
Peter
The short answer is "No, the process is not the same in western longitudes." The good news is that it is some what easier to do and less confusing.
The Greenwich Hour Angle (GHA) of the body is how far west the body's geographic position (GP) was from the Greenwich meridian at the time of the sight.
Your Dead Reckoning (DR) longitude is (approximately) how far eastward or westward you were from the Greenwich meridian at the time of the sight.
The Local Hour Angle (LHA) is how far westward the body's GP was from your longitude, or from your nearby chosen AP's longitude.
All "Hour Angles" of any sort are always only measured westward from their starting point all the way around to 360°
If you are east of Greenwich you have to add your eastern longitude to the GHA of the body to compute how far westward the body's GP was from yourself. Additionally, just for using tables or taking an exam, the LHA result has to come out to be a whole number of degrees. To work that out we first find the GHA of the body at the time of the sight. Then we subtract the minutes and tenths of the GHA that we found from 60.0' to find a value for the minutes and tenths to append to our DR whole degree of longitude to create an AP longitude that will make the addition and the resulting LHA come out to be a whole number of degrees. We know full well that we are not at the AP longitude that we then use. We only contrived it in order to make the tables work -- or to get the same answer as on the exam.
When you are in western longitudes you will be subtracting your longitude from the Greenwich Hour Angle to determine how far westward the GP of the body was from you at the time of the sight. Once again, even in western longitudes, for using the tables or taking and exam, you need the result to come out to be a whole number of degrees. But this time all you need to do is to take the minutes and tenths of the GHA as you found it and append that value unchanged to your whole degree of DR longitude to contrive your AP longitude. Now when you subtract the longitude of your AP position from the GHA of the body to find your LHA the minutes and tenths will disappear to 00.0' leaving you with a whole degree result.
The final check (only really important for taking exams) is to confirm in either case that your resulting AP longitude is within 30.0' of longitude from your DR position, and if not to adjust the whole degree value of your AP by one degree while leaving the minutes and tenths alone so that it is -- just so your results will match the exam question answer.
Hope this helps -- let me know if you have more questions.
Peter